[Asterisk-Users] Freak incidents, who's to blame?

[Peter Svensson]

On Tue, 3 May 2005, Andrew Kohlsmith wrote:

> On May 3, 2005 02:22 pm, Ryan Courtnage wrote:
> >  From what I've read, glare is common in 2-way loopstart (kewlstart)
> > circuits, and is impossible(?) to eliminate completely.  But now I'm
> > wondering what Nortel would tell a customer who experiences glare on
> > their new Meridian system... they must do something to prevent glare
> > from happening.  Any ideas?
> 
> Nope.
> 
> Technically it shouldn't be possible with PRI but it is and does happen.  
> Typically you hunt "up" starting at the highest available channel, and the 
> telco hunts "down" which tends to keep it at bay until things get busy.

Glare is when both the net and the cpe end attempt to seize a line
simultaneously and both believe they succeeded. Glare really is impossible
on a pri as a B channel can not be requested and allocated to both parties
by mistake. The handshaking performed leaves no ambiguity as to which call
a line is allocated to.

However, a similar situation can occur when the cpe end requests a
specific B channel in a SETUP message instead of leaving the channel
selection to the net end. Unlike the glare condition this situation is
detected and the net end prevails. The cpe end should then try to allocate 
another B channel with a new SETUP message. 

Unfortunatly Asterisk as a cpe device neither lets the net end allocate
the B channel, nor does it retry using a different B channel. The problem 
is that Asterisk does not see the whole PRI as a single link with several 
channels, it sees the inidvidual channels with a common signalling path. A 
specific B channel is allocated before the signalling starts. This is a 
deficiency in Asterisk, not in isdn in general.

The solution for Asterisk is the same as for glare-prone links - hunt for
channels in the opposite direction. Note that on isdn links quite a few 
operators will by default _not_ hunt from one end or another, this has to 
be requested. The convention then is for the net end to hunt low-to-high 
and the cpe end to hunt high-to-low.

Finally, even on isdn you have end devices (phones) which may themselves 
be prone to the human equivalent of glare - picking up the handset before 
the ring is heared. Some phones allow the user to request an outside line 
by pressing a button to prevent this.

Peter